We will complete the charaterization of thermolabile ts mutants of VSV containing lesions in M or G protein and correlate virion properties with those of the isolated mutant proteins, in pure form and in reconstituted lipid-protein vesicles. We will use these mutants to characterize cell surface receptors which lie in the pathway of productive infection, and to study subsequent intracellular events occurring prior to primary transcription. We will determine the role of each of the internal viral proteins in the process of viral budding by using mutants in conjunction with fluorescence photobleaching recovery to measure mobility of newly synthesized G protein on infected cell surfaces at permissive and non-permissive temperatures, and correlate results wth freeze-drying electron microscope observation of the cell surfaces. We will attempt to define all requirements for viral assembly, cellular as well as viral, by using temperature sensitive mutants of VSV to develop a cell-free viral budding system.